Analysis of Corrosion and Perforation of Seawater Pipelines in Fuqing Nuclear Power Auxiliary Cooling Water System
There are many seawater pipelines in a nuclear power plant. As seawater with high corrosive property, in spite of anti-corrosion measures such as coating, rubber lining and sacrificial anode have already applied to pipelines since the design, but corrosion occurs endlessly, especially perforation of the pipelines. An analysis on the case history of the corrosion induced perforation of pipeline of the auxiliary cooling water system at Fuqing nuclear power plant has been made, and it follows that the corrosion case may be ascribed to the poor construction quality, the insufficient protection supplied by sacrificial anode and the galvanic corrosion induced by the coupling of the perforated pipe with the conjoint heat exchanger. The findings and conclusions may be a useful reference for the selection of corrosion protection countermeasures as well as the further design and construction of the seawater pipeline.
|Key words: seawater pipeline corrosion perforation galvanic corrosion sacrificial anode|
QIAO Ze, ZHAO Xingbao, CHEN Ping, LIN Jiankang
The Auxiliary Cooling Water System (SEN) is an important system for conventional islands in nuclear power plants. Its function is to provide vacuum pump sealed water heat exchangers for conventional island closed circulating cooling water systems (SRI) and condenser vacuum systems (CVI). The cooling water (sea water) is finally discharged into the seawater by the heat dissipation of each user in the closed water system and the sealed water heat exchanger of the vacuum pump. Under normal operating conditions, the two pipes are taken from two circulating water inlets, and the electric water filter and the water and water heat exchanger are both operated and one standby. When a circulating jellyfish tube is not available, the corresponding electric cooling butterfly valve on the auxiliary cooling water take-off branch pipe is automatically closed, and the cooling water is continuously supplied by the other circulating water main pipe. When the two main pipes are not available, the SRI system and the CVI system are continuously provided. It will lose its function due to the loss of cooling water, which will eventually lead to shutdown and shutdown.
On February 28, 2016, the 2SRI101RF (a heat exchanger of the SRI system) was found on the site of the operation of the Fuqing nuclear power plant. The seawater side outlet valve 2SEN009VC (a valve of the SEN system) broke and cracked near the upstream flange weld. The size is approximately 30 mm x 15 mm.
1 Corrosion status evaluation
After the short pipe was disassembled, the corrosion appearance of the inner wall of the pipe was examined as a whole. For the internal coating defects and corrosion of the short tube of the SEN system, the overall evaluation was carried out according to the national standard .
Corrosion profile of the inner wall of the short pipe is shown in Figure 1a. It shows a typical corrosion zone. The coating on both ends of the flange is corroded seriously. There is no obvious corrosion spot in about 1/3 of the middle, but a small amount of coating is damaged. Consistent with the situation of grinding to remove surface corrosion products and paint, as shown in Figure 1b, the etch pit is dense near the flange and the corrosion pit is slightly in the middle.
The fracture site is located near the joint flange of the 2SRI101RF heat exchanger. The size of the fracture is about 112 mm × 50 mm. The specific shape is shown in Figure 2.
Nearly 1/3 of the flange on the exit side of the 2SRI heat exchanger is severely corroded. The side flange and the short pipe near the SRI heat exchanger are socket welded. After welding, there is a step of about 10 mm × 15 mm. This position is surface treatment. And coating weak points, under the impact of flowing sea water, coating failure is easy to occur. The flange sealing surface is severely corroded, and the corrosion at individual locations spreads along the gap between the short tube and the flange. The overall corrosion of the flange near the 2SEN009VC side is relatively slight, and there is no serious corrosion defect on the flange surface of the flange surface, but there are also corrosion spots of different sizes on the inner side. The main reason is that the flange sealing surface is uncoated, and the seawater enters the gap between the flange and the gasket, causing corrosion of the flange portion and expanding along the metal substrate at the bottom of the coating.
Fig.1 Corrosion morphologies of 2SEN009VC pipeline inwall: (a) original corrosion morphology, (b) morphology after rust cleaning
Fig.2 Corrosion morphologies of perforation; (a) original corrosion morphology of perforation, (b) morphology of perforation after rust cleaning
2 Analysis of corrosion causes
2.1 Anti-corrosion design review
By reviewing the SEN system design file, the upstream of the breakpipe is connected to the 2SRI101RF heat exchanger outlet flange and downstream to the 2SEN009VC. The perforated short tube length is 351 mm, the specification is DN900, φ920X10, the material is Q235-A, the flange material on the side connected to the heat exchanger is 16MnII, and the joint flange on the 2SEC009VC side is made of carbon steel.
According to the “30-H500201S-J1303A circulating water system pressure pipe construction manual”, the inner wall of the SEN system steel pipe adopts the joint anti-corrosion measures of high performance coating + sacrificial anode cathodic protection, and the paint design protection period is 20 a. The coating system for the inner wall of the steel pipe is as follows: the primer is a general-purpose wear-resistant epoxy paint (dry film thickness 450 μm/two lanes), the intermediate paint is ethylene epoxy paint (dry film thickness 40 μm/one), and the topcoat is Wuxi Self-polishing antifouling paint (dry film thickness 310 μm/piece), designed to a total film thickness of 800 μm, the coating system is mainly used for anti-corrosion coating on the bottom of hull, and has also been widely used in the nuclear power seawater system in Fujian and Fujian. A sacrificial anode (35 kg sacrificial anode block) is placed on the straight tube below the downstream bend of the 2SEN009VC valve connected to the perforated short tube, and the straight tube is 1800 mm high. Calculated at 1/2 position, the distance from the anode to the etched perforation is approximately 3880 mm.
Corrosion-resistant materials are used as anti-corrosion measures on the contact side of the short-tube upstream SRI heat exchanger and seawater. Specifically, the heat exchange tubes are seamless titanium tubes B338, and other parts in contact with seawater are lined with pure titanium. Q345R+TA2 is used for the head, barrel section (seawater side), tube sheet and nozzle (φ920 mm × (12+3) mm).
By reviewing the above design information and combining with the on-site corrosion investigation, it can be found that the short tube and its vicinity have the following problems or hidden dangers in the anti-corrosion design:
(1) Sacrificial anodes have very limited protection against short tubes. The short pipe and the upstream SRI heat exchanger are not electrically insulated by an insulating flange. The ordinary flange connection used directly, after field measurement, the resistance between the short pipe flange and the seawater outlet flange of the SRI heat exchanger is 0.2. Ω, the electrical connection between the two is good, with electronic channel conditions for galvanic corrosion. The data show [2,3] that the galvanic sequence and potential difference between the two are very large. The self-corrosion potential of carbon steel in flowing seawater is -0.4 V (SHE), and the self-corrosion potential of titanium is 0.15 V (SHE). ), and the area of the short tube is much smaller than the area of the titanium in the heat exchanger (heat exchange area of 1650 m2), which is adjacent to each other. Therefore, the tendency of galvanic corrosion between the “large cathode” of the heat exchanger titanium and the “small anode” of the short tube occurs very high. A sacrificial anode is placed downstream of the short tube, and its protection range is about 2 m. However, after bending, the maximum protection distance of the sacrificial anode can only reach about 1 m, which is far from the short tube perforation area (the distance from the sacrificial anode to the perforation position is about 3880 m), and the heat exchanger cannot be offset. The galvanic corrosion of titanium on short tubes. In addition, when the 2SRI heat exchanger is in standby, the only protection of the sacrificial anode block is completely shielded.
(2) Risk of crevice corrosion on flange sealing surfaces. The on-site investigation found that the flange sealing surfaces at both ends of the short pipe were not used for anti-corrosion coating treatment, and the relevant corrosion protection measures were not found in the relevant design documents provided. In other words, the flange sealing surface lacks basic corrosion protection measures. Under operating conditions, seawater can flow or penetrate into the flange sealing surface through the coating boundary. When the gasket is aged and damaged, it is more likely to cause seawater retention and accumulation in the sealing surface area, thereby causing crevice corrosion.
Table 1 Measuring result of coating thickness
2.2 Anti-corrosion coating quality evaluation
The coating thicknesses of the regions 1, 2, and 3 in Fig. 3 were measured, and the results are shown in Table 3-3. The thickness of the inner coating of zone 2 is approximately 1000 μm, while the thickness of the inner zone of zone 1 and zone 2 is approximately 370-550 μm. The coating thickness of Zone 1 and Zone 3 is significantly lower than the design criteria.
Fig.3 Coating’s measuring area of the perforatingpipeline
Since the coating system has undergone significant aging, more details on corrosion protection construction have been difficult to trace.
2.3 Analysis of corrosion causes
Based on the results of the on-site investigation, the review of the anti-corrosion design documents, and the traceable anti-corrosion coating quality inspection results, it can be seen that: (1) The short-walled coating has been seriously affected during the approximately 2 years of operation. Deterioration. From the fact that this coating system deteriorates prematurely, and the coating thickness of the weld zone is significantly lower than the design requirements and the main area inside the short pipe, it is inferred that the short pipe is insufficient in the anticorrosive coating construction process. In the case where the thickness of the coating system is significantly lower than the design requirements, oxygen and water can reach the metal matrix through the micropores of the coating, resulting in premature electrochemical corrosion of the metal matrix. Some studies  have shown that the thickness of the coating is directly proportional to the corrosion resistance of the coating. If the coating thickness is not up to standard or the surface treatment before coating does not meet the requirements, it may cause premature coating failure and metal matrix corrosion. (2) According to the drawings and documents, the protection range of the downstream sacrificial anode cannot reach the short pipe area, and the strong galvanic corrosion effect of the titanium alloy on the short tube of the heat exchanger is very good in the case of poor coating quality. It is easy to cause corrosion perforation in a short time. In fact, the galvanic corrosion of the heat exchanger titanium to the surrounding metal is very large. (3) The reason for the obvious corrosion of the short-tube flange sealing surface is that no anti-corrosion measures are applied at this place, and the base metal is directly exposed to the infiltrated seawater, resulting in the formation of an occlusion cell and crevice corrosion.
3 Conclusions and measures
Comprehensive analysis and analysis can be found that the inner wall of the short pipe does not fully consider the galvanic corrosion effect of the heat exchanger titanium on the carbon steel material in the anti-corrosion design, and the sacrificial anode arranged downstream can not protect the short pipe area, and the anti-corrosion Problems such as the quality of the coating caused the short tube to preferentially corrode in the weak area of the inner wall and eventually caused corrosion perforation.
The improvement measures are as follows: (1) Insulation treatment between the outlet flange of the heat exchanger and the short pipe, insulating the galvanic action of the heat exchanger titanium on the connected metal (downstream short pipe, valve, etc.), and sealing the short pipe together with the seal The lining protection shall be carried out together with the lining. The lining shall cover the flange sealing surface to avoid crevice corrosion. (2) Reconfirm the arrangement of the downstream sacrificial anode and reconside the layout scheme with the confirmation result to make it to the upstream valve and The valve back pipe can play a good protective role; (3) Conduct a systematic anti-corrosion design review and on-site corrosion assessment of the relevant seawater system, check for missing traps, prevent micro-duration, and avoid more and more serious corrosion problems. occur.
The authors have declared that no competing interests exist.
Source: China Seawater Pipelines Manufacturer – Yaang Pipe Industry (www.metallicsteel.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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